Simple reasoning helps us to see who the wild beast is.
First Revelation does not stand alone in the explanation of beasts.
Daniel chapters 7 and 8 are very clear who BEASTS represent - it even gives names of ruling powers of the general time frame for different beasts.
So very plainly the beasts DO represent various political systems throughout the earth.
EACH HEAD represents a segment or remnant of the past powers - that is why there are 7 heads - Egypt, Assyria, Babylon, Medo-Persia, Greece, Rome, and the two horned power of today's influence - USA/Britain.
As for a "universal system" - how fragile the global integration is for finances, environmental decisions, even shifting friendships/enemies.
At this point, the UN is not on the same level of influence as the political powers of today. It lends itself to a different path/goal. But it continues to keep a world-wide contact through relief and troops around the globe. There are very few if any countries who have not been contacted in some way with the works of the UN and recognize its reach to the far corners of countries - small and great.
We have to remember that Revelation has SEVERAL beasts - and it needs to be clear what beast is being referred to .
When you say that Jehovah's Witnesses believe that the "beast" is the UN, are you talking about the 7 headed beast? Or the Scarlet colored "beast"? Or the two horned "beast" referred to as a "false prophet", or is it the "image of the "beast"?
So which "wild beast" will devour the harlot?
Revelation 17:16 adds another layer to what has been said - the verse says that it is the "TEN HORNS and the wild beast" that will hate the harlot and will make her devastated and naked.
Who are the TEN HORNS? That is your clue as to who the "wild beast is."
Go back to Daniel chapter 7 and see the comparison of the ten horns.